Receiver For Use In An Ultra-Wideband Communication System

ABSTRACT

In an ultra-wideband (“UWB”) receiver, a received UWB signal is periodically digitized as a series of ternary samples. The samples are continuously correlated with a predetermined preamble sequence to develop a correlation value. When the value exceeds a predetermined threshold, indicating that the preamble sequence is being received, estimates of the channel impulse response (“CIR”) are developed. When a start-of-frame delimiter (“SFD”) is detected, the best CIR estimate is provided to a channel matched filter (“CMF”) substantially to filter channel-injected noise.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part of application Ser. No.13/775,282, filed 25 Feb. 2013 (“First Parent Application”).

The First Parent Application is a Continuation-In-Part of applicationSer. No. 13/033,098, filed 23 Feb. 2011 (“Second Parent Application”),which is in turn related to Provisional Application Ser. No. 61/316,299,filed 22 Mar. 2010 (“Parent Provisional”).

The First Parent Application is also a Continuation-In-Part ofapplication Ser. No. 12/885,517, filed 19 Sep. 2010, now U.S. Pat. No.8,437,432, issued 7 May 2013 (“Parent Patent”), which is in turn alsorelated to the Parent Provisional.

The subject matter of this Application is also related to the subjectmatter of PCT Application Serial No. PCT/EP2013/070851, filed 7 Oct.2013 (“Related Application”).

This application claims priority to:

-   1. The First Parent Application;-   2. The Second Parent Application;-   3. The Parent Patent;-   4. The Parent Provisional; and-   5. The Related Application;    collectively, “Related References”, and hereby claims benefit of the    filing dates thereof pursuant to 37 CFR §1.78(a)(4).

The subject matter of the First and Second Parent Applications, theParent Patent, the Parent Provisional, and the Related Application(collectively, “Related References”), each in its entirety, is expresslyincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to ultra-wideband communicationsystems, and, in particular, to a receiver for use in an ultra-widebandcommunication system.

2. Description of the Related Art

In general, in the descriptions that follow, we will italicize the firstoccurrence of each special term of art which should be familiar to thoseskilled in the art of ultra-wideband (“UWB”) communication systems. Inaddition, when we first introduce a term that we believe to be new orthat we will use in a context that we believe to be new, we will boldthe term and provide the definition that we intend to apply to thatterm. In addition, throughout this description, we will sometimes usethe terms assert and negate when referring to the rendering of a signal,signal flag, status bit, or similar apparatus into its logically true orlogically false state, respectively, and the term toggle to indicate thelogical inversion of a signal from one logical state to the other.Alternatively, we may refer to the mutually exclusive boolean states aslogic_(—)0 and logic_(—)1. Of course, as is well known, consistentsystem operation can be obtained by reversing the logic sense of allsuch signals, such that signals described herein as logically truebecome logically false and vice versa. Furthermore, it is of norelevance in such systems which specific voltage levels are selected torepresent each of the logic states.

In general, in an ultra-wideband (“UWB”) communication system, a seriesof special processing steps are performed by a UWB transmitter toprepare payload data for transmission via a packet-based UWB channel.Upon reception, a corresponding series of reversing steps are performedby a UWB receiver to recover the data payload. Details of both series ofprocessing steps are fully described in IEEE Standards 802.15.4(“802.15.4”) and 802.15.4a (“802.15.4a”), copies of which are submittedherewith and which are expressly incorporated herein in their entiretyby reference. As is known, these Standards describe required functionsof both the transmit and receive portions of the system, but specifyimplementation details only of the transmit portion of the system,leaving to implementers the choice of how to implement the receiveportion.

One or more of us have developed certain improvements for use in UWBcommunication systems, which improvements are fully described in thefollowing pending applications or issued patents, all of which areexpressly incorporated herein in their entirety:

“A Method and Apparatus for Generating Codewords”, U.S. Pat. No.7,787,544, issued 31 Jul. 2010;

“A Method and Apparatus for Generating Codewords”, application Ser. No.11/309,222, filed 13 Jul. 2006, now abandoned;

“A Method and Apparatus for Transmitting and Receiving ConvolutionallyCoded Data”, U.S. Pat. No. 7,636,397, issued 22 Dec. 2009;

“A Method and Apparatus for Transmitting and Receiving ConvolutionallyCoded Data”, application Ser. No. 12/590,124, filed 3 Nov. 2009, and theIssue Fee on which was paid on 11 Dec. 2012; and

“Convolution Code for Use in a Communication System”, application Ser.No. 13/092,146, filed 21 Apr. 2011.

One particular problem in multi-path, spread-spectrum systems, includingUWB, is channel-induced noise present in the received signal. One commontechnique for significantly reducing the noise level relative to thereceive level is to develop, during reception of a training sequenceportion of the preamble of each transmitted packet, an estimate of thechannel impulse response (“CIR”). Following detection in the receivedpacket of the start-of-frame delimiter (“SFD”), the best CIR estimate isreversed in time and the complex conjugate is developed. This conjugateCIR estimate is thereafter convolved with the payload portion of thepacket using a channel matched filter (“CMF”). Shown in FIG. 1 is a UWBreceiver 10 adapted to operate in this manner. As is known, the signalreceived via an antenna 12 is continuously conditioned by a filter 14.During reception of the training sequence, channel estimator 16 developsfrom the conditioned signal the conjugate CIR estimate. During receptionof the payload data, detector 18 employs a CMF (not shown) to convolvethe conditioned signal with the conjugate CIR estimate, therebysignificantly improving the signal-to-noise ratio (“SNR”) andfacilitating recovery of the payload data. See, also, “EfficientBack-End Channel Matched Filter (CMF)”, U.S. Pat. No. 7,349,461, issued25 Mar. 2008.

As noted in 802.15.4a, §5.5.7.1, “UWB devices that have implementedoptional ranging support are called ranging-capable devices (RDEVs).”(Emphasis in original.) For certain applications, such RDEVs arecommonly implemented in the form of a relatively compact, autonomousradio-frequency identification (“RFID”) tag or the like. Due to thesmall form factor and limited power supply, it is especially importantto select circuit implementations that provide maximum performance atminimum power. Unfortunately, in known implementations of the UWBreceiver, improvements in performance usually come at the expense ofpower. For example, it is known that a rake filter provides goodperformance in multi-path, spread-spectrum systems such as UWB. See,e.g., slide 21 of “The ParthusCeva Ultra Wideband PHY Proposal”, IEEEP802.15 Working Group for Wireless Personal Area Networks, March 2003, acopy of which is submitted wherewith and which is expressly incorporatedherein in its entirety by reference. However, known rake filterimplementations tend to consume significantly more power than otherprior art techniques.

While it has been proposed to implement the front-end of aspread-spectrum receiver using a fast, 1-bit analog-to-data converter(“ADC”) to reduce the size (in terms of transistor count) of theconvolution logic in both the CIR estimator and the CMF, suchimplementations are known to be particularly sensitive tocontinuous-wave (“CW”) interference. This CW interference can besubstantially rejected using a full 2-bit, sign+magnitude implementationsuch as that described by F. Amoroso in “Adaptive A/D Converter toSuppress CW Interference in DSPN Spread-Spectrum Communications”, IEEETrans. on Communications, vol. COM-31, No. 10, Oct. 1983, pp. 1117-1123(“Amoroso83”), a copy of which is submitted wherewith and which isexpressly incorporated herein in its entirety by reference. However, insuch implementations, having dual representations of the 0-state, i.e.,[−0, +0], tend to increase system entropy, resulting inless-than-optimal circuit/power efficiency.

We submit that what is needed is an improved method and apparatus foruse in the receiver of a UWB communication system to filterchannel-induced noise. In particular, we submit that such a method andapparatus should provide performance generally comparable to the bestprior art techniques while requiring less circuitry and consuming lesspower than known implementations of such prior art techniques.

BRIEF SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of our invention, we provideapparatus for use in an ultra-wideband (UWB) communication system inwhich multi-symbol packets are transmitted via a transmission channel,each transmitted packet comprising a multi-symbol preamble. Inaccordance with our invention, the apparatus comprises: a preambledetect adapted sequentially to develop first and second CIR estimatesover a sliding window comprising respective sets ofmost-recently-received preamble symbols; a CIR estimate store adaptedselectively to store a CIR estimate; and a correlator adapted to: storethe first CIR estimate in the CIR estimate store; compute the scalarproduct of the second CIR estimate with the conjugate of the storedfirst CIR estimate; and if the resultant scalar product determines thatthe second CIR estimate insufficiently resembles the stored first CIRestimate, store the second CIR estimate in the CIR estimate store;otherwise, provide a preamble detect signal. Preferably, the preambledetect is adapted to: develop the first CIR estimate over a slidingwindow comprising a first predetermined number of a first set ofmost-recently-received preamble symbols; and the second CIR estimateover a sliding window comprising a second predetermined number of asecond set of most-recently-received preamble symbols, the second setbeing consecutive to the first set.

In one embodiment, the above apparatus is implemented as a receiver foruse in a UWB communication system.

We also provide a method for operating a UWB communication system inwhich multi-symbol packets are transmitted via a transmission channel,each transmitted packet comprising a multi-symbol preamble, Inaccordance with our method, we develop a first CIR estimate over asliding window comprising a first predetermined number of a first set ofmost-recently-received preamble symbols; store the first CIR estimate;develop a second CIR estimate over a sliding window comprising a secondpredetermined number of a second set of most-recently-received preamblesymbols, the second set being consecutive to the first set; compute thescalar product of the second CIR estimate with the conjugate of thestored first CIR estimate; and if the resultant scalar productdetermines that the second CIR estimate insufficiently resembles thestored first CIR estimate, we store the second CIR estimate; otherwise,we provide a preamble detect signal.

In one embodiment, the above method is practiced in a receiver speciallyadapted for use in a UWB communication system.

In each of our embodiments, we prefer to employ ternary samples, butother sample sizes, including binary, may be employed in appropriateapplications.

We submit that each of these embodiments of our invention filterchannel-induced noise as effectively as any prior art method orapparatus now known to us, while consuming less power.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Our invention may be more fully understood by a description of certainpreferred embodiments in conjunction with the attached drawings inwhich:

FIG. 1 illustrates, in block diagram form, a prior art receiver adaptedfor use in a UWB communication system;

FIG. 2 illustrates, in block diagram form, one embodiment of thereceiver shown in FIG. 1, but constructed in accordance with ourinvention;

FIG. 3 illustrates, in flow diagram form, operation of the correlatorblock shown in FIG. 2;

FIG. 4 illustrates, in block diagram form, a more detailedimplementation of the correlator block shown in FIG. 2;

FIG. 5 illustrates, in block diagram form, a more detailedimplementation of the multiplexor shown in FIG. 4;

FIG. 6 illustrates, in block diagram form, a more detailedimplementation of each of the several correlators shown in FIG. 4;

FIG. 7 illustrates, in block diagram form, a more detailedimplementation of the accumulator block shown in FIG. 2;

FIG. 8 illustrates, in block diagram form, a more detailedimplementation of each of the several accumulators shown in FIG. 7;

FIG. 9 illustrates, in block diagram form, a more detailedimplementation of the windowing block shown in FIG. 2;

FIG. 10 illustrates, in block diagram form, a more detailedimplementation of each of the several energy detectors shown in FIG. 9;

FIG. 11 illustrates, in block diagram form, a more detailedimplementation of the preamble detect block shown in FIG. 2;

FIG. 12, comprising FIG. 12 a, FIG. 12 b, and FIG. 12 c, illustrates, inblock diagram form, alternate, more detailed implementations of thesum-of-products portion of the SFD detect block shown in FIG. 2;

FIG. 13 illustrates, in block diagram form, a more detailedimplementation of the SFD detection portion of the SFD detect blockshown in FIG. 2;

FIG. 14 illustrates, in block diagram form, an alternate embodiment ofthe receiver shown in FIG. 1, but constructed in accordance with ourinvention;

FIG. 15, comprising FIG. 15 a, FIG. 15 b, FIG. 15 c, and FIG. 15 d,illustrates, in block diagram form, alternate, more detailedimplementations of the correlator and accumulator blocks shown in FIG.14;

FIG. 16, comprising FIG. 16 a, FIG. 16 b, FIG. 16 c, and FIG. 16 d,illustrates, in block diagram form, alternate, more detailedimplementations of the windowing and CIR interpolation blocks shown inFIG. 14;

FIG. 17 illustrates, in block diagram form, a more detailedimplementation of the CMF block shown in FIG. 2;

FIG. 18 illustrates, in block diagram form, a more detailedimplementation of the CMF phase filter blocks shown in FIG. 17;

FIG. 19 illustrates, in schematic form, one exemplary implementation ofthe complex multipliers, c_([0:127]), within the CMF phase filter blocksshown in FIG. 18;

FIG. 20 illustrates, in block diagram form, an alternate embodiment ofthe correlator block shown in FIG. 2;

FIG. 21 illustrates, in block diagram form, a more detailedimplementation of each of the several correlators shown in FIG. 20; and

FIG. 22 illustrates, in flow diagram form, operation of one embodimentof the preamble detect block shown in FIG. 2.

In the drawings, similar elements will be similarly numbered wheneverpossible. However, this practice is simply for convenience of referenceand to avoid unnecessary proliferation of numbers, and is not intendedto imply or suggest that our invention requires identity in eitherfunction or structure in the several embodiments.

DETAILED DESCRIPTION OF THE INVENTION

Shown in FIG. 2 is a UWB receiver 10′ constructed in accordance with ourinvention. As in the prior art system shown in FIG. 1, the signalreceived by antenna 12 is continuously conditioned by filter 14. Theconditioned signal is then periodically sampled by an analog-to-digitalconverter (“ADC”) 20 and provided as a continuous series of digitalsamples. In accordance with a preferred embodiment of our invention, ADC20 is specially adapted to provide each digital sample in ternary form,i.e., [−1, 0, +1]. In view of the difficulty of currently availablestandard digital circuit technology efficiently to represent a 3-valuevariable in the form of a single ternary trit, we anticipate, at leastin the near term, such variables will require representation using 2conventional, binary bits, wherein a first one of the bits representsthe numeric component of the variable, i.e., [0, 1], and the second bitrepresents the sign of the variable, i.e., [+, −]. In this regard, itcould be argued that circuit technology has not progressed all that muchsince Soviet researchers built the first (perhaps only?) documentedternary-based computer systems. See, “A Visit to Computation Centers inthe Soviet Union,” Comm. of the ACM, 1959, pp. 8-20; and “SovietComputer Technology—1959”, Comm. of the ACM, 1960, pp. 131-166; copiesof which are submitted herewith and which are expressly incorporatedherein in their entirety by reference.

In the context of our invention, our trit can be distinguished from aconventional sign+magnitude implementation such as that described inAmoroso83, cited above. Consider the strategy for A/D conversion shownin FIG. 5 of Amoroso83; and, note, especially, that there are threeseparate and distinct switching thresholds: (i) a sign threshold [T₀];(ii) a positive magnitude threshold [T₀+Δ]; and (iii) a negativemagnitude threshold [T₀−Δ]. (See, also, Amoroso83, p. 1119, lines21-24.) We have discovered that adapting the ADC to use ONLY a positivemagnitude threshold [T₀+Δ] and a negative magnitude threshold [T₀−Δ]results in only a very small loss in resolution, while improving theperformance of an impulse radio UWB receiver. Accordingly, in ourpreferred embodiment, ADC 20 implements only positive/negative magnitudethresholds [T₀±Δ], thereby simplifying the circuit while simultaneouslyimproving both the conversion time of the ADC 20 and, in general, theperformance of the receiver. Such an implementation lends itselfnaturally to our trit-based scheme, wherein the three defined statesindicate, for example, that:

-   -   [−1]=>the input is below the negative magnitude threshold        [T₀−Δ];    -   [0]=>the input is between the negative magnitude threshold        [T₀−Δ] and the positive magnitude threshold [T₀+Δ]; and    -   [+1]=>the input is above the positive magnitude threshold        [T₀+Δ].        In contrast to a conventional sign+magnitude implementation, our        trit-based ADC 20 can be readily adapted to operate either at a        higher sample rate (improved performance but with more power) or        at an equivalent sample rate (substantially equivalent        performance but with less complexity, thereby reducing both        circuit size and power consumption).

Upon power-on, a switch 22 will be configured to direct the trit samplestream to a correlator 24 portion of channel estimator 16′. In oneembodiment, correlator 24 is adapted to correlate the sample stream withthe known training sequence, and periodically to provide a partialfinite impulse response (“FIR”) for each symbol. An accumulator 26 isprovided to accumulate the partial FIRs on a per-symbol basis for someor all of the symbols comprising the synchronization header (“SHR”).

Windowing 28 is provided to selectively develop a CIR estimate based ona selected, sliding subset, i.e., window, of the accumulated per-symbolFIRs. When a sufficient number of per-symbol FIRs have been accumulated,windowing 28 develops an initial CIR estimate 30. In one embodiment,windowing 28 is adapted thereafter to periodically develop new CIRestimates as symbols slide through the window.

A preamble detect 32 correlates each new CIR estimate with the CIRestimate 30. In the event that preamble detect 32 determines that thenew CIR estimate sufficiently resembles CIR estimate 30, then preambledetect 32 signals that the preamble has been detected. If, however, thenew CIR estimate does not sufficiently resemble the CIR estimate 30,preamble detect 32 stores the new CIR estimate as CIR estimate 30. Inone embodiment, preamble detect 32 is adapted to reset accumulator 26each time a new CIR estimate 30 is stored, thereby facilitatingdevelopment of the CIR estimate 30 using only trit samples from selectedportions of the preamble of the received packet.

As is known, the predefined SFD code comprises a predetermined set ofN_(SFD) symbols. Once a predetermined minimum number of symbols havebeen received and continuing for each subsequent preamble symbol, an SFDdetect 34 correlates the SFD detection code with the accumulated FIRs ofthe N_(SFD) most recently received symbols. In one embodiment, SFDdetect 34 is adapted to configure switch 22 so as to direct the tritsample stream to a CMF 36 portion of detector 18′ when the SFD detectioncorrelation exceeds a selected threshold, indicating that the full SHRhas been received and the PHY header is immediately to follow.

In accordance with our invention, the CIR estimate 30 as of the momentof SFD detection comprises the best estimate of the impulse response ofthe channel. In one embodiment, windowing 28 is adapted to provide anindex indicative of the portion of accumulator 26 upon which the finalCIR estimate 30 was based. In effect, the index indicates the portion ofthe accumulator containing the most energy, which, in most cases, alsocontains the path with the highest energy, i.e., the peak path. In aranging application, the portion of accumulator 26 immediately precedingthe index can be analyzed, e.g., using interpolation, to identify thedirect path.

In one embodiment, CMF 36 is adapted to correlate the received tritsample stream with the final, i.e., best, CIR estimate 30, therebyfiltering the CIR noise from the sample stream. The filtered samplestream is then processed in a known manner by De-hop 38, De-spread 40,Viterbi 42 and Reed-Solomon (“RS”) decode 44 to recover the datapayload.

FIG. 3 illustrates, in flow diagram form, the general method ofoperation of the UWB receiver 10′ illustrated in FIG. 2 as describedabove.

In one embodiment, correlator 24 may be implemented as a poly-phasecorrelator. For example, in a 500 MHz UWB system oversampled by 2 timesthe chip rate, the ADC sample rate must be 1000 MHz. Using aconventional single-phase correlator, the correlator must also run at1000 MHz. However, if, as shown in FIG. 4, we employ 16 parallelcorrelators_([1:16]), each may now run at 62.5 MHz. For a spreading codeof, say, length 127 (upsampled by 8 to give a preamble symbol length of1016 samples), a mux 46 may be employed to selectively distribute thetrit samples to each of the correlators_([1:16]); for a differentspreading code, say, length 31 (upsampled by 32 to give a preamblesymbol length of 992 samples), mux 46 is not necessary.

Various alternate embodiments will occur to those skilled in this art.For example, if, in the embodiment shown in FIG. 4, only 8 parallelcorrelators are implemented, each must now run at 125 MHz. In such anembodiment, the length 127 spreading code would still be upsampled by 8to give a preamble symbol length of 1016 samples, whereas the length 31spreading code would be upsampled by 32 to give a preamble symbol lengthof 992 samples. If, however, oversampling is performed at 4 times thechip rate, then it may be desirable to implement 32 parallelcorrelators, each now running at 62.5 MHz. In such an embodiment, thelength 127 spreading code would be upsampled by 16 to give a preamblesymbol length of 2032 samples, whereas the length 31 spreading codewould be upsampled by 64 to give a preamble symbol length of 1984samples. However, if only 16 parallel correlators are implemented, eachmust now run at 125 MHz. In this embodiment, the length 127 spreadingcode would still be upsampled by 16 to give a preamble symbol length of2032 samples, whereas the length 31 spreading code would be upsampled by64 to give a preamble symbol length of 1984 samples. Thus, it will beappreciated that the number of phases and the operating rates thereofcan be varied to accommodate the desired operating characteristics ofthe UWB system.

In the embodiment illustrated in FIG. 4, and in each of the variantsthereof described immediately above (as well as many others), mux 46 maybe implemented using a delay line of appropriate length, as shown inFIG. 5.

In one embodiment, each of the correlators (see, FIG. 4) may beimplemented as shown in FIG. 6. As illustrated, odd and even numberedtrit samples received from mux 46 are sequentially staged through odddelay chain d_([1:127]) and even delay chain d_([2:126]), respectively.Periodically, the stored trit samples are multiplied in parallel byrespective phase-specific coefficients via multipliers f_([1:127]). Asis known, the coefficients are related to the known preamble sequence.In accordance with our invention, these coefficients will have one of 3values: [−1, 0, +1]. Accordingly, each of the multipliers f_([1:127])may be of simple form. Accumulators 48 a and 48 b sum the partialproducts developed by multipliers f_([1:127]), and a summer 50 developsthe full correlator output.

In one embodiment, accumulator 26 may be implemented as a poly-phaseaccumulator. For example, as illustrated in FIG. 7, for cooperation withthe embodiment of correlator 24 shown in FIG. 4, accumulator 26 may beimplemented as a corresponding number of parallel accumulators_([1:16]),each operating at the same frequency as the correlators_([1:16]). A mux52 may be employed to selectively distribute the FIR estimates developedby correlators_([1:16]) to each of the accumulators_([1:16]).

In one embodiment, each of the accumulators (see, FIG. 7) may beimplemented as shown in FIG. 8. As illustrated, as FIR estimates, b_(x),are periodically received from mux 52, summer 54 develops a summation,which is then recirculated through a 64-element delay chain s_([1:64])back to summer 54, thereby allowing the summer 54 to continuouslydevelop a partial CIR estimate, c_(x).

We have noted that, in many cases, the accumulator is significantlylonger than the CMF requires to perform its function. One option toreduce the length of the CIR estimate is to implement a windowingmechanism adapted to identify the portion of the accumulator having thegreatest energy. In the embodiment illustrated in FIG. 9, we haveincluded windowing 28, wherein each of the partial CIR estimates, c_(x),is applied to a respective one of a plurality of energydetectors_([1:x]). A summer 56 periodically develops a summation of thedetected energy, and, if the sum is greater than a previously storedmaximum, Max_(n), a comparator 58 will replace the previously storedmaximum with the current sum; simultaneously, the current sample index,i_(x), will be stored as Index_(n). In accordance with our invention,Index_(n) always indicates the position in the received sample stream atwhich the detected energy attained Max_(n).

In one embodiment, each of the energy detectors_([1:x]), may beimplemented as illustrated in FIG. 10. In accordance with our invention,a calculator 60 periodically develops an estimate, e_(x), of the energydetected in the most recent CIR estimate, each of which is thenforwarded to a subtractor 62 for accumulation. Simultaneously, eachestimate, e_(x), is forwarded via an n-element delay chain, d_([1:n]),to subtractor 62 for subtraction from the current accumulation. Thus,the accumulated change in detected energy, Δe_(x), represents thedetected energy measured over the n most-recently-received CIRestimates. As illustrated in FIG. 10, calculator 60 can be adapted todevelop the estimate, e_(x), as a maximum sum of energies, ( )², or as amaximum sum of magnitudes, | |. Whereas the maximum sum of energies maybe appropriate for detecting both the preamble and the SFD, the requiredsquaring operation may be expensive to implement; one possiblealternative is to implement this function as a look-up table (“LUT”). Athird possible approach may be to develop each estimate, e_(x), usingpeak path loss windowing; this approach may be particularly advantageousfor direct path detection.

In one embodiment, preamble detect 32 may be implemented as illustratedin FIG. 11. In accordance with our invention, preamble detect 32 isadapted to develop a new CIR estimate 30 a over a sliding windowcomprising a predetermined number, say, 8, of the most-recently-receivedpreamble symbols. A correlator 64 periodically computes the scalarproduct of the new CIR estimate 30 a with the conjugate of a storedprior CIR estimate 30. If the resultant scalar product determines thenew CIR estimate 30 a insufficiently resembles the stored CIR estimate30, the new CIR estimate 30 a is stored. This process is repeated onlyuntil the correlation first exceeds a predetermined resemblancethreshold selected to correspond to a sufficiently good correlationbetween the two consecutive CIR estimates. As would be expected, theindex, Index, corresponding to the stored CIR estimate is also stored(see, FIG. 9), thereby facilitating identification of the start of thepreamble in the received sample stream.

In one embodiment, the preamble detect 32 can be adapted to determineCIR estimate resemblance in a manner similar to the following pseudocodealgorithm:

  [Pcode 1] DECLARE:  $vAcc; // accumulator (complex vector of length1024)  $vCor; // correlation (complex vector of length 1024)  $i; // FORloop index (integer)  $j; // WHILE loop index (integer)  $rResemblance;// measure of the resemblance of two   channel estimates (real) $vSaved; // saved 128 biggest accumulator samples   (complex vector oflength 128)  $rThreshold; // resemblance threshold (real)  $iWindow; //start of window index (integer) START;  // Get a first crude estimate ofthe channel:   // Note this will only be a channel estimate if a  preamble is being received.  FOR ($i = 1; $i <= 8; $i++)  {   $vCor =Correlation of received signal with reference    preamble sequence;  $vAcc += $vCor;   $i++;  }  $j = 0; // initialize WHILE  WHILE ($j ===0)  {   $vSaved = Window the accumulator to find sequence of    128biggest samples; // Save the windowed portion of    $vAcc   $iWindow =index of start of window;   // Get a second crude estimate of thechannel:    // Note this will only be a channel estimate if a    preamble is being received.   $vAcc = 0;   FOR ($i = 1; $i <= 8;$i++)   {    $vCor = Correlation of received signal with     referencepreamble sequence;    $vAcc += $vCor;    $i++;   }   Core $vAcc; // corethe accumulator   $rResemblance = 0.0;   FOR ($i = 1; $i <= 128; $i++)  {    $rResemblance += $vSaved[$i] × $vAcc[$i +     $iWindow]*; // *denotes complex conjugate    $i++;   }   IF ($rResemblance_(real) >$rThreshold)   {    $j = 1; // end WHILE   }  }  Flag that Preamble hasbeen detected; END;

In the foregoing pseudocode algorithm, we have proposed that theaccumulator be cored before the resemblance threshold is applied. As isknown, coring is a technique adapted to reduce noise falling below apredetermined threshold. In this embodiment, we propose to apply acoring threshold proportional to the standard deviation of the noise inthe accumulator, σ_(noise). We choose to assume that the signal iscorrupted by additive white Gaussian noise (“AWGN”):

σ_(noise)=√{square root over (N _(acc)2σ_(corr) ²)}

where:

-   -   N_(acc)=Number of accumulated symbols; and    -   σ_(corr) ²=Variance of the correlator output.

In this embodiment, the correlator output variance is a function of thenumber of non-zero values in the preamble code and the probability of anon-zero in the ADC output:

σ_(corr) ² =N _(nz) P _(nz)

where:

-   -   N_(nz)=No. non-zeros in preamble code; and    -   P_(nz)=Probability of a non-zero in ADC output.

For a 1-bit ADC, P_(nz)=1, and, for a 1.5-bit ADC, P_(nz) depends on thegain in the adaption algorithm, but it will typically be ⅓.

For a preamble detection algorithm that accumulates 8 symbols beforecomparing with the previous 8 accumulated symbols (N_(acc)=8), andassuming that the coring threshold is twice the standard deviation ofthe noise, the coring threshold can be determined as follows:

TABLE 1 Example coring thresholds for preamble detection, PreambleCoring Code Length Nnz ADC Bits Pnz σ_(corr) ² σ_(noise) threshold 31 161 1 16 16 32 31 16 1.5 1/3 16/3 9.2 18.4 127 64 1 1 64 32 64 127 64 1.51/3 64/3 18.5 37

In general, we recommend that coring be done any time the channelestimate is used for something, including:

-   -   During preamble detection, just before correlating the two noisy        channel estimates to see how similar they are (e.g., as        described above);    -   After preamble reception, just before transferring the CIR to        the CMF; and    -   Just before interpolating the first paths in the channel        estimate to find the peak of the leading path.

Note that the best coring threshold to use in each of thesecircumstances will usually be different.

In one embodiment, as illustrated in FIG. 12 and FIG. 13, SFD detect 34may be implemented as a sum-of-products (“SoP”) calculator 66, an SoPadder 68 and windowing 70. As illustrated in FIG. 12 a, each SoPcalculator 66 may be adapted to develop the SoP by summing the productof a respective one of the CIR estimates, c_(x), and the respectivedelayed FIR estimate, s_(x) (see, FIG. 8). A simpler, but somewhat lesseffective, alternative is to develop the SoP as the sum of the CIRestimates, c_(x), but wherein the signs of which are first modified bythe sign of the respective delayed FIR estimate, s_(x), as illustratedin FIG. 12 b. A third approach, a hybrid of the embodiment shown in FIG.12 b but which should be more immune to noise effects, is illustrated inFIG. 12 c, wherein each CIR estimate, c_(x), is first squared, e.g.,using a suitable LUT. This square is then multiplied by the sign of therespective CIR estimate, c_(x). Prior to summing, the sign of eachsquare is modified by the sign of the respective delayed FIR estimate,s_(x). In accordance with our invention, SFD detect 34 initiatesoperation only after the accumulators 26 have processed at least N_(SFD)symbols, and, thereafter on a symbol-by-symbol basis, correlates themost-recently-received window of N_(SFD) symbols with a predeterminedcode, p_([1:8]); when the correlation first exceeds a predeterminedthreshold, SFD detect 34 signals that the SFD has been detected. It canbe shown that setting p_([1:8]) to be the SFD code itself, maximizes thecorrelation at the point when the SFD is received. It may be preferable,however, to select p_([1:8]) so as to maximize the difference betweenthe correlation output at SFD detect time and the maximum correlationoutput before this, i.e., while receiving the first parts of the SFD;such p_([1:8]) can easily be found by an exhaustive search.

In one other embodiment, channel estimator 16″ may be implemented asshown in FIG. 14. In this form, the trit input samples are developedperiodically by a set of 16 ADCs_([1:16]), which, because they areconnected in parallel, may be operated at a substantially lower clockrate than the single ADC of the embodiment shown in FIG. 2. Each of thetrit sample streams is then used by a respective one of 16correlator-accumulators_([1:16]) to develop partial FIR estimates.Windowing 28′ develops the CIR estimate from a selected, sliding set ofthe partial FIR estimates. From the windowed partial FIR estimates, SFDdetect 34′ detects the SFD.

In one embodiment, variations of which are illustrated by way of examplein FIG. 15, the accumulator portions of correlator-accumulators_([1:16])may be implemented as 16 accumulators, A_([1:16]), each comprising 1616-bit accumulators, which may be selectively configured as 16 parallelslices of 16 16-bit accumulators, A_([1]):A_([2])-A_([14]):A_([15]), inhigh-bandwidth mode or as 8 parallel slices of 32 16-bit accumulators,A_([1])-A_([15]), on low bandwidth mode. By way of example, FIG. 15 aillustrates an implementation suitable for operating at relatively lowbandwidth in a system wherein each symbol comprises 1016 samples,whereas FIG. 15 b illustrates an implementation suitable for operatingat a higher bandwidth wherein each symbol comprises 2032 samples.Similarly, FIG. 15 c illustrates an implementation suitable foroperating at relatively low bandwidth in a system wherein each symbolcomprises 992 samples, whereas FIG. 15 d illustrates an implementationsuitable for operating at a higher bandwidth wherein each symbolcomprises 1984 samples. In accordance with our invention, theillustrated embodiments are adapted to operate on trits, as indicated bythe use of the symbol “1” in FIG. 15. As we did above, we indicate asingle-trit delay element as d_(x), a preamble symbol coefficientmultiplier by c_(x), and a multi-bit sample delay-chain element bys_(x).

Alternative implementations of windowing 28′ are illustrated in FIG. 16.In particular, FIG. 16 a illustrates an implementation suitable foroperating at relatively low bandwidth in a system wherein each symbolcomprises 1016 samples, whereas FIG. 16 b illustrates an implementationsuitable for operating at a higher bandwidth wherein each symbolcomprises 2032 samples. Similarly, FIG. 16 c illustrates animplementation suitable for operating at relatively low bandwidth in asystem wherein each symbol comprises 992 samples, whereas FIG. 16 dillustrates an implementation suitable for operating at a higherbandwidth wherein each symbol comprises 1984 samples.

In one embodiment, CMF 36 may be implemented as a poly-phase channelmatched filter. As noted above, in a 500 MHz UWB system oversampled by 2times the chip rate, the ADC sample rate must be 1000 MHz. Using aconventional single-phase CMF, the CMF must also run at 1000 MHz.However, if, as shown in FIG. 17, we employ 16 parallelfilters_([0:15]), each may now run at 62.5 MHz.

Since the SFD detect 34′ will identify the SFD one or two symbols priorto the actual end of the SFD, CMF 36 will have sufficient time to loadthe CIR estimate into a CIR estimate block. By way of example, the CIRestimate block may comprise a long shift-register adapted to seriallyreceive and store each of the CIR coefficient pairs. Following the endof the SFD, payload data bits, a_(x), are continuously received andsequenced through a delay line comprising 9 16-bit delay elements,X_([0:8])[0:15].

As shown in FIG. 18, each of the filters_([0:15]) comprises a128-complex-sample delay line, d_([0:127]), each delay stage having arespective complex multiplier, c_([0:127]), and a summer. After each setof 16-samples of payload data, the multipliers in each filter develop,in parallel, respective 5-bit partial sums as a function of respectiveCIR coefficient pairs, and the summer develops a respective resultvector value, Y_([0:15]), as a function of the resultant 128 partialsums. Although the summer may be implemented as a single, largecarry-save adder (“CSA”) (fast but relatively circuit intensive), ourpoly-phase implementation of the CMF 36 allows us to instantiate thesummer as a hierarchy of smaller carry-propagate adders (“CPA”) (slowerbut less circuit intensive). Preferably, after rounding, each resultvector value, Y_([0:15]), comprises 5-bits.

In one other embodiment, as the real and imaginary CIR coefficient pairsare clocked into the CMF estimate block, it is possible to transformthem into sum and difference terms, essentially pre-computing theresults of the complex multiplication. These terms may then be stored inregisters, thereby allowing the complete complex multiplication to bereplaced by a single 9-way multiplexor, as shown in the following table:

Dor Doi Result Real Result Imaginary −1 −1 −Cr + Ci −Cr − Ci −1 0 −Cr−Cr −1 1 −Cr − Ci −Cr + Ci 0 −1 Ci −Ci 0 0 0 0 0 1 −Ci Ci 1 −1 Cr + CiCr − Ci 1 0 Cr Cr 1 1 Cr − Ci Cr + CiAlternatively, they can be computed dynamically; a circuit adapted toimplement the real result portion of this function is shown by way ofexample in FIG. 19. As will be clear to those skilled in this art, theimaginary portion of this function can be readily implemented withidentical circuitry by simply reversing the connections to the +1 and −1inputs of the Ci mux. We expressly intend that the term multiplier, asused in this specification and the appended claims, include alternateembodiments that accomplish the desired multiplication function,including, for example, the multiplexor-based embodiment shown in FIG.19. Indeed, those skilled in this art will readily recognize that ouruse of trit-based sampling greatly facilitates replacement oftraditional high-complexity, time-critical, dynamic multiplicationcircuits with substantially lower-complexity, less time-criticalmultiplexor-based alternatives.

Shown in FIG. 20 is an alternate embodiment of the correlator block 24of FIG. 2. In this embodiment, each of the polyphase correlators may beinstantiated using the embodiment shown in FIG. 21. We believe that thisalternate embodiment may provide benefits in some applications over theembodiments described above.

Shown in FIG. 22 is a flow for an alternate embodiment of the preambledetect block 32 of FIG. 2. In normal operation, once a preamble isdetected, the receiver 10′ will start to search for the SFD. During theSFD search, as described in the Related Applications, receiver 10′ alsostarts carrier recovery and a short time later timing recovery. However,many things could go wrong at the start of the SFD search. Inparticular, preamble detection may have been triggered wrongly or thecarrier recovery loop may fail to lock. If these and other errors arenot detected, time and energy will be wasted until the SFD search timesout.

In order to prevent this, we require preamble confirmation before theSFD search is initiated. In a valid preamble, except where noise orunstable carrier and timing loops cause errors, all preamble symbolsshould be demodulated as +1. To confirm the preamble, we count thenumber of +1s during a predetermined number of symbols, X. If the countexceeds a threshold, Y, then the preamble is deemed to have beenconfirmed. If preamble is not confirmed within X symbols, the preambleis deemed to have been detected in error and is rejected; and receiver10′ resumes the preamble search. Upon preamble confirmation, the timingand carrier loops are deemed to have settled, and the search for the SFDis initiated. At this point, it may be advantageous to adjust selectedinternal configuration settings within receiver 10′, e.g., the SFDcoring thresholds can be raised. We submit that advantages of doing thisrather than, say, using a fixed count before starting the SFD searchinclude:

-   -   1. A lower preamble detection threshold can be chosen because a        bad preamble detect will be rejected, whereas the other way a        bad preamble detect will cause receiver 10′ to waste time and        energy by searching for an SFD that may never come;    -   2. In good receiver conditions, e.g., high SNR, the SFD search        can be started earlier; this may be important if the preamble is        a relatively short one; and    -   3. Receiver 10′ may adjust it's configuration parameters as soon        as it is well enough stabilized, increasing the likelihood of        detecting SFD without an error (false detect or missed detect).

As an example, in one implementation, we require at least 11 of out of16 symbols demodulated during the SFD timeout period to be +1 to confirmpreamble. If this is the case, we assume that a real preamble wasdetected and that the carrier loop and timing loops are convergingproperly.

We believe that this alternate embodiment may provide benefits in someapplications over the embodiments described above.

Although we have described our invention in the context of twoalternative embodiments, one of ordinary skill in this art will readilyrealize that many modifications may be made in such embodiments to adapteither to specific implementations. By way of example, it will take butlittle effort to adapt our invention for use with a 1-bit ADC schemewhen it can be anticipated that the target application will not besubject to significant levels of in-channel CW interference. Further,the several elements described above may be implemented using any of thevarious known semiconductor manufacturing methodologies, and, ingeneral, be adapted so as to be operable under either hardware orsoftware control or some combination thereof, as is known in this art.

Thus it is apparent that we have provided an improved method andapparatus for use in the receiver of a UWB communication system tofilter channel-induced noise. In particular, we submit that our methodand apparatus provides performance generally comparable to the bestprior art techniques while requiring less circuitry and consuming lesspower than known implementations of such prior art techniques.Therefore, we intend that our invention encompass all such variationsand modifications as fall within the scope of the appended claims.

What we claim is:
 1. Apparatus for use in an ultra-wideband (UWB)communication system in which multi-symbol packets are transmitted via atransmission channel, each transmitted packet comprising a multi-symbolpreamble, the apparatus comprising: a preamble detect adapted todevelop: a first CIR estimate over a sliding window comprising a firstpredetermined number of a first set of most-recently-received preamblesymbols; and a second CIR estimate over a sliding window comprising asecond predetermined number of a second set of most-recently-receivedpreamble symbols, the second set being consecutive to the first set; anda CIR estimate store adapted selectively to store a CIR estimate; and acorrelator adapted to: store the first CIR estimate in the CIR estimatestore; compute the scalar product of the second CIR estimate with theconjugate of the stored first CIR estimate; and if the resultant scalarproduct determines that the second CIR estimate insufficiently resemblesthe stored first CIR estimate, store the second CIR estimate in the CIRestimate store; otherwise, provide a preamble detect signal.
 2. Theapparatus of claim 1 wherein the correlator is further characterized asdetermining the resemblance of the first CIR estimate and the second CIRestimate by comparing the scalar product to a predetermined resemblancethreshold selected to correspond to a sufficiently good correlationbetween the first and second CIR estimates.
 3. The apparatus of claim 1wherein the preamble detect is further adapted to develop, beforedeveloping said first CIR estimate: a count of a third predeterminednumber of preamble symbols that comprise a selected value; and wherein,said first CIR estimate is developed only if said count is at least afourth predetermined number.
 4. In an ultra-wideband (UWB) communicationsystem in which multi-symbol packets are transmitted via a transmissionchannel, each transmitted packet comprising a multi-symbol preamble, themethod comprising the steps of: developing a first CIR estimate over asliding window comprising a first predetermined number of a first set ofmost-recently-received preamble symbols; storing the first CIR estimate;developing a second CIR estimate over a sliding window comprising asecond predetermined number of a second set of most-recently-receivedpreamble symbols, the second set being consecutive to the first set;computing the scalar product of the second CIR estimate with theconjugate of the stored first CIR estimate; and if the resultant scalarproduct determines that the second CIR estimate insufficiently resemblesthe stored first CIR estimate, storing the second CIR estimate;otherwise, providing a preamble detect signal.
 5. The method of claim 4further characterized as determining the resemblance of the first CIRestimate and the second CIR estimate by comparing the scalar product toa predetermined resemblance threshold selected to correspond to asufficiently good correlation between the first and second CIRestimates.
 6. The method of claim 4 further characterized as: developinga count of a third predetermined number of preamble symbols thatcomprise a selected value; and wherein, said first CIR estimate isdeveloped only if said count is at least a fourth predetermined number.